int128
===========

    .. code-block:: CPP

        using i128 = __int128;

        std::ostream &operator<<(std::ostream &os, i128 n) {
            std::string s;
            while (n) {
                s += '0' + n % 10;
                n /= 10;
            }
            std::reverse(s.begin(), s.end());
            return os << s;
        }